1. Field of the Invention
The present invention relates to multi-carrier wireless communication systems. More particularly, the present invention relates to signals which can be transmitted on a plurality of different carrier frequencies.
2. Background to the Invention
Multi-carrier wireless communication systems (including networks), in which signals can be transmitted from a transmitter to a receiver on a plurality of different carrier frequencies, are known. In such systems, information to be transmitted is typically distributed over a number of the available carrier frequencies. The distribution by the system of the information (in the simplest terms, determining which carrier frequency or frequencies to use at a particular time) is made according to (but not only) the results of measurements made on link qualities. More particularly, the system needs the link qualities as inputs, but also takes into account other inputs like, for example, the load on each carrier frequency in order to decide how to distribute the frequencies to the users. These link quality measurements are to be made during a so-called scanning interval. In this scanning interval, a respective test or pilot signal is sent from the transmitter to the receiver on each carrier frequency and in each of a sequence of time slots. The strengths of the transmitted signals are known, and the strengths of the corresponding received signals are measured, giving an indication of the path loss for each carrier frequency and in each time slot of the scanning interval.
The path loss for each carrier frequency typically comprises three components: a distance-dependent path loss which is simply a function of the distance between the transmitter and receiver; a shadowing or shadow fading component determined by the environment between the transmitter and receiver (in other words, dependent upon what obstacles, if any, there are between the transmitter and receiver); and a fast fading component which results in fluctuations in received signal strength over time scales typically in the range of a few tens to a few hundreds of microseconds. The measurements of received signal strength thus give an indication of the quality of the communication link between the transmitter and the receiver in each carrier-time bin, and the results of the measurements can be used, if desired, to give a value indicative of an average quality of the link on each different carrier frequency over the measurement interval (i.e. the scanning interval). These average quality indications can then be used to decide how to allocate radio resources. For example, a channel for which the measured quality over the scanning interval is high may be used in preference to one for which the measured quality during the scanning interval is low. In this context, a high channel quality may indicate that the received signal to noise ratio over the scanning interval is large, and low quality may indicate that the signal to noise ratio is relatively small. The allocation of radio resources may, of course, be more sophisticated than simply deciding whether to use a particular carrier frequency (or carrier band) or not, and suitable methods for allocating resources according to the results of test measurements in the scanning interval will be apparent to the person skilled in the relevant art.
Thus, in conventional multi-carrier systems, the determination of link qualities for use in determining how to allocate radio resources has necessitated taking a large number of measurements, in particular one measurement for each of the carrier-time bins in the scanning interval. This was because the decisions concerning the allocation of radio resources involves all of the available carrier frequencies, and so measurements were required on the qualities of links on each of the available carrier frequencies. While this conventional technique of taking measurements in each of the carrier-time bins of the scanning interval provided a good indication of link qualities, it was expensive in terms of measurement resources because it required measuring all of the carrier frequencies and all of the time slots. Moreover, this measurement burden (i.e. the number of measurements to be made, and the corresponding amount of processing required) increases with the number of different carrier frequencies that the multi-carrier system is adapted to use.
Accordingly, there is a need for an improved apparatus and method for estimating qualities of links between a transmitter and a receiver in multi-carrier wireless communication systems that can be achieved by making fewer measurements than conventional techniques.